Generally, conventional fuel injection valves for internal combustion engines include a needle and a spring which are operated such that if the pressure of fuel supplied to the valve is higher than the opening pressure of the valve, a valve flow passage opens, and if it is not, the valve flow passage remains closed. When high-pressure fuel is supplied from a fuel pump to the fuel injection valve, if pressure higher than the opening pressure is formed in the fuel injection valve, the pressure of fuel oil pushes the needle upwards while overcoming the force with which the spring pushes the needle downwards. Thereby, the valve flow passage opens. Then, fuel is injected into a cylinder through several nozzle holes which are formed in an end of a nozzle. In this injection method, all of the nozzle holes are opened or closed according to a single injection mechanism with a single preset opening pressure. Therefore, in low-load operation in which injection pressure of fuel is comparatively low, conditions of fuel spray deteriorate, whereby the combustion efficiency is reduced.
In the case of a dual-fuel engine that can alternate between diesel fuel operation and gas fuel operation, when the engine is in a diesel mode in which diesel fuel is used to operate the engine, fuel is injected into a combustion chamber by a conventional mechanical fuel pump and a fuel injection valve. On the other hand, in a gas mode in which the engine is operated by gas fuel, a separate electronic control fuel injection system (a common rail fuel injection system) is typically used for pilot injection in which only a small amount of fuel appropriate to function as an ignition source of a gas mixer is injected along with compressed air supplied into the combustion chamber.
Introduced as representative examples of fuel injection valves of the convention dual-fuel engines, a method of MAN Co. and a method of Wartsilla Co. will be described below. In the method of MAN Co., main injection operation for a diesel mode using a mechanical fuel pump and pilot injection operation for a gas mode using a common rail fuel injection system are performed by respective fuel injection valves. In this case, there is a problem in that the structure of a cylinder head is complicated due to installation of the fuel injection valves.
In the case of the method of Wartsilla Co., so as to simplify the installation structure of the fuel injection valves, needles and nozzle holes for main injection operation for a diesel mode using a mechanical fuel pump and pilot injection operation for a gas mode using a common rail fuel injection system are installed and arranged in a row in a single fuel injection valve such that the main injection and the pilot injection can be conducted by the single fuel injection valve. However, this case is problematic in that, even when only either the main injection operation or the pilot injection operation malfunctions or an element related to either of them is damaged, the entirety of the fuel injection valve must be replaced with a new one.